Humans aren’t the only primates who binge on sweet food. In the presence of an abundant supply of fruit, orang-utans will stuff themselves with wild berries, durians and figs.

University of Sydney scientist David Raubenheimer says there was a good reason for the primate’s greed, a discovery that also offered an insight into why one in three adult humans were overweight.

In Borneo forests, the availability of fruit was highly unpredictable prompting the animals to overeat when it was in season.

To adapt to this boom and bust food cycle, orang-utans’ bodies had evolved sophisticated mechanisms to store surplus energy as fat that could be drawn upon later, said Professor Raubenheimer, a nutritional ecologist at the Charles Perkins Centre.

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What excited Professor Raubenheimer most about this discovery was that it mimicked the way humans also overate sweet foods and stored sugar as fat.

Professor Raubenheimer, who is preparing the results for publication, said it was likely early humans evolved this physiological adaptation when they lived in an environment where sweet foods like wild honey were only available some of the time.

Scarcity of these foods prompted a preference for those nutrients, he said.

But as these eating behaviours had developed a long time ago, and could not be directly observed, scientists had had to rely on indirect evidence from anthropology and archaeology to support their theories.

‘‘The exciting thing about orang-utans is that we’ve got an example of this happening in a living primate,’’ he said.

In Borneo, Rutgers University primatologist Erin Vogel and her team had spent seven years observing 49 orang-utans. They had painstakingly recorded every mouthful of food eaten, measured the nutrient content of each meal and collected urine samples to understand the link between the animal’s eating habits and their environment.

Professor Raubenheimer said the difference between orang-utans and humans was that we had used our considerable brainpower to alter the environment and the types of food available.

‘‘I predict that if orang-utans had the cultural know-how they would be fat fruit farmers,’’ he said.

Over the past 10,000 years people had learned to cultivate carbohydrate rich crops and domesticate animals with a higher fat content than wild beasts. And the industrial revolution spurred the mass production of highly processed food.

Professor Raubenheimer said the combination of high-energy food being available year-round with our predilection for sugar had culminated in the obesity epidemic.

‘‘We lay down fat but never have the need to subsequently draw on it,’’ he said.

‘‘We’ve manipulated our ecology at a much faster rate than our genetic evolution has been able to respond.’’